Spice – How critical is it to store spices in constant temperature

Dried herbs really do only last around six months, certainly no longer than a year. They're generally easy to get in small quantities, though.

Spices are trickier. They'll generally last rather longer, but the time will vary. If the spice is used for the colour and/or heat (e.g. turmeric, chilli) it will generally last much, much longer than one used for its smell and/or taste. Whole spices last much, much longer than pre-ground; I generally buy whole spices from asian stores, they sell them in much larger amounts for the same prices as supermarkets; and use a coffee mill to grind the right amount just before cooking (and keep a separate one for coffee!).

If kept in the dark, in a dry, clean, airtight glass container, spices and dried herbs will certainly not become health hazards, and should not pick up unpleasant tastes or odours. The strong-tasting / strong-smelling ones will, however, lose taste and smell, eventually almost completely; since that is the whole point of using them, you might as well not bother once they're off.

As others have said, there are few spices with umami. However, if you're looking for something that you can use in the same way as a spice, then I suggest simply blitzing dried porcini (cep) mushrooms in a blender or grinder into a fine powder and using that. It has a deep umami flavour - try rubbing it on a steak before cooking and you'll be blown away.